The present invention relates to rocker arms for use in valve control systems of internal combustion engines, and more particularly, to rocker arms of the type which are formed from a flat metal blank.
Rocker arms are commonly used in internal combustion engines as part of a valve control system, i.e., a system which is able to control the movement (opening and closing) of an engine poppet valve. Typically, the rocker arm is utilized to transmit motion from a rotating camshaft, and specifically, from a cam lobe, to the engine poppet valve, by means of the rocker arm. As is well known to those skilled in the art, there is a substantial load applied by the “lift portion” of the cam lobe to the rocker arm, as the engine poppet valve is being moved to the open position, typically in opposition to the biasing force of a valve return spring.
Among the more common type of valve control systems are those in which the rocker arm is of the “center-pivot” type, in which the rocker arm pivots about a centrally located rocker shaft while one axial end of the rocker arm engages the engine poppet valve, and the cam lobe engages the axially opposite end of the rocker arm. Another common type of valve control system is one in which the rocker arm is of the “end-pivot” type, in which one end of the rocker arm pivots about a generally stationary member (such as a hydraulic lash adjuster), the axially opposite end engages the engine poppet valve, and the cam lobe engages a cam follower or cam contact surface disposed intermediate the axially opposite ends of the rocker arm. The above-described valve control system is typically referred to as an OHC (overhead cam) system. There could also be other rocker arm types, beside the center-pivot and end-pivot types, but most rocker arms in commercial use comprise one of these two types. However, it should be understood that, although the OHC type valve control system is the preferred embodiment, the present invention is not necessarily limited to any one particular type of rocker arm, in terms of the pivot location of the rocker arm.
Some known valve control systems for engine poppet valves utilize only a single rocker arm, but in recent years, those skilled in the art have developed a number of various valve control systems which utilize two separate rocker arms which may, selectively, be “latched” to move in unison, or may be “unlatched” to be pivotable relative to each other. By way of example only, such a valve control system, utilizing two rocker arms, may provide “dual lift” in which, for example, when the rocker arms are latched, one cam lobe engages one of the rocker arms to provide high lift of the engine poppet valve, whereas, when the rocker arms are unlatched, a different cam profile engages the other rocker arm to provide a low lift of the engine poppet valve. Examples of such dual lift, latchable rocker arm valve control systems are shown in U.S. Pat. Nos. 5,524,580; 5,584,267; and 5,655,488, all of which are assigned to the assignee of the present invention and incorporated herein by reference.
Although the present invention may be utilized to provide an improved rocker arm for use in a single rocker arm valve control system, or to provide either the inner or the outer rocker arm in a dual rocker arm system, it is especially advantageous when used to provide the outer, high lift rocker arm in a dual lift system, and will be described in connection therewith.
One of the conventional, prior art methods utilized to manufacture rocker arms is to form the “rough” rocker arm by any one of a number of well known processes, such as investment casting, and then to finish-machine all of the various important surfaces, dimensions, etc. Rocker arms made in this manner typically have excellent mechanical properties, but are also normally quite expensive, especially considering that, by way of example, a six cylinder engine would require at least twelve rocker arms, and in some cases, as many as eighteen or twenty-four rocker arms.
In many engine applications, the rocker arms are formed as steel stampings, in order to reduce substantially the manufacturing cost of each rocker arm. However, for many engine valve control systems, a stamped rocker arm may not possess the mechanical properties required for that particular valve control system especially in the case of dual rocker arm systems. Such a stamped rocker arm may be especially insufficient in regard to the cam follower surface of the rocker arm, i.e., the surface which engages the lift portion of the cam lobe during engine poppet valve “lift”.